GEOTHERMAL RESOURCES IN THE JIDONG AREA, NORTH CHINA BASIN WANG Yanyun1, LIU Shuyi1 and PANG Zhonghe2 1Jidong Oil Exploration and Development Company, 51-A Lubei District, Tangshan, Hebei Province, China, 063004 2Institute of Geology and Geophysics, Chinese Academy of Sciences, P. O. Box 9825, Beijing, China, 100029 Key Words: geothermal resources, geothermal utilization 2500 to 4000m, and forming three depressions: Changli, Jidong oil field, Tangshan, North China Basin Leting and Jianhe. ABSTRACT Seven other faults are the West Laowangzhuang, East Laowangzhuang, Xinanzhuang, Bogezhuang, Hongfangzi, During a long period of petroleum exploration, plenty of low- Zhangjiapu and Leting-Luannan faults, which delimits the medium temperature geothermal water resources have been numerous uplifts and depressions. discovered in dozens of drill-holes in the Jidong Oil Field, North China Basin. There are five main thermal abnormal The Gaoliu fault, located in the Nanpu depression, is only zones in the Jidong area with potential for geothermal developed in the cover layers. It traverses the Nanpu resources development: the Matouying uplift, Begezhuang- depression from the Xinanzhuang fault in the west, to the Xinanzhuang uplift, Laowangzhuang-Tian-zhuang uplift, Baigezhuang fault in the east. It acts as a main thermal source Jianggezhuan uplift and the Gaoliu fracture zone in the Nanpu fault because of the intense movement at the end of the depression. The total surface area of these five zones is about Cenozoic era. 2 1900 km . The total recoverable heat from the hot water has 3. MAJOR THERMAL AQUIFERS been estimated to be 2.0 x 1018J. Upon the crystalline basement of the area, there are four sets 1. INTRODUCTION of sedimentary formations, i.e. Protozoic, Paleozoic, Mesozoic and Cenozoic strata. Thermal aquifers are found in The exploration scope of the Jidong Oil Field covers mainly all these strata. However, due to extension and thickness, only on land areas to the south of Yanshan Mountain and off shore the sandstone formations in upper Tertiary and limestone strata in lower Paleozoic serve as practical thermal water Bohai Bay. Its land area is about 3600 km2 which involves aquifers for geothermal development in the area. partially both Tangshan city and Qinghuangdao city of Hebei Province (Fig. 1). The upper Tertiary includes the Guangtao formation (Ng) and Minghuazheng formation (Nm) which are aluvial sandstone A network of railways and country roads facilitates formations with interbeded sandstone and mudstones. The transportation in the area. Jingtang port on the east coast is a total thicness varies from 1000 to 2400m. major port in northern China. In Tangshan region there are abundant natural resources, such as coal, oil and gas etc; and a The Lower Paleozoic includes the Majiagou formation (Om) variety of industries including steel, power generation, pottery and Fujunshan formation (Ef), both composed of limestone and porcelain crafts. Being called “granary of Jidong”, wheat, and dolomite. Fractures and even caves are developed in the corn, rice, cotton, peanut, vegetables, fruits are grown here. rocks that can form good aquifers. This has also been Stockbreeding and aquaculture are well developed, too. evidenced by drill-holes. Tourism has blossomed in Jidong in recent years due to some 4. GEO-TEMPERATURE PATTERN famous historic sites and beautiful summer resorts, such as the Great Wall at Shanhaiguan, Nandaihe, Beidaihe and Golden Beach summer resorts, and many others. A geothermal anomaly in the Jidong field is defined as an area where the geothermal gradient is greater than 3.5°C/100m or 2. REGIONAL GEOLOGICAL SETTING heat flow density is higher than 62mW/m2. The total area of thermal anomalies is about 480 km2. The occurrence of these The area is a part of the Bohaiwan Meso-Cenozoic anomalies is controlled by geological structures. sedimentary basin. Three groups of major faults, namely NE, NW, and EW fault systems, control the formation and 4.1 High Geothermal Anomalies Correspond to Uplifts evolution history of the basin. The general structural pattern consists of six pairs of uplifts and depressions according to As was described in Chen (1988) for the whole North China the thickness of the Meso-Cenozoic strata. They are Nanpu, Basin (the study area of this paper is in the northmost sector Jianhe, Leting, Shijiutuo, Changli and Qinhuangdao of the basin), most of the geo-temperature anomalies can be depressions; Xihe, Laowangzhuang, Xinanzhuang, related to the occurrence of pre-Cenozoic basement uplifts. In Bogezhuang, Matouying and Jianggezhuang uplifts (Fig. 2). northern part of the study area, a low temperature zone is formed along the piedmont of the Yanshan fold belt, which is The Yanshan fold-belt is to the north of the area, seperated dominated by cooler southbound groundwater flow seeping from the Bohaiwan Basin by the Ninghe-Changni Fault, the into the northern part of the area. largest fault in the region. The displacement of basement rock is up to 1500-3000m, with the upthrown basement rocks The higher geo-temperature anomalies occur in basement buried from 500 to 1000m, and the downthrown buried from 1941 Wang, Y. et al. uplift area, such as Xihe, Laowangzhuang, Bogezhuang, Jianggezhaung and Matouying uplifts, as well as Tanghai Salinity of the thermal water is quite low in the area. By “buried hill”. The geo-temperature gradient of the vertical sequence, in the upper Tertiary, the salinity is sedimentary cover is 3.5-3.7 °C/100m, with corresponding between 0.43 and 2.25 g/l, the majority of which is 0.8-1.5 g/l heat flow 62-65 mW/m2, and the temperature is up to 46-50 (based on data of 86 drill-holes). The salinity of the Ordovician is 0.8-2.95 g/l, and the Cambrian is 1.28-4.01g/l °C at 1000m depth. (12 drill-holes). The tendency is for the salinity to rise with increased depth, but not with increased temperature. In contrast, in the lower zones in depressions, the geo- ° temperature gradient in the sedimentary cover is 2.9–3.1 C - - 2 Cl and HCO3 are the major ions in the thermal water and /100m, with corresponding heat flow 50–55 mW/m , and the 2- temperature is 42-44°C at 1000m. In deeper parts of increase with salinity. SO4 is low and not affected by depressions, for example, the western part of the Leting salinity. K+ and Na+ are major ions and they increase linearly depression, geothermal gradient is only 2.7-2.8 °C /100m, and ++ ++ ° with salinity. Mg and Ca are low and not affected by the temperature at 1000m is less than 40 C. As for the salinity. transitional zones, geo-temperature gradient and deep temperature are between the two cases as discussed above. The chemical content of thermal water from Ng and the lower The general framework of this geo-temperature pattern is 2- + summarized in Fig. 3. section of Nm is low in SO4 and high in Na . The water chemical types are HCO3-Na, HCO3-Cl-Na, Cl-HCO3-Na 4.2 Vertical Variation in Gradients is Controlled by and Cl-Na. Lithology Based on samples from 16 drill-holes, the average salinity of Lithology determines the thermal conductivity of rocks and thermal water in the lower Paleozoic is 1.57 ± 0.19 g/l. Their sediments, and therefore controls the geo-temperature chemical types are HCO3-Na, HCO3-Cl-Na and Cl-HCO3- gradients. In the Cenozoic cover, thermal gradient can be Na. related to the ratio of sandstone to mudstone, higher in mud After comparing the major chemical contents and trace sections, and lower in sandstone sections. Taking the lower elements of the thermal water to China National Standard for section of Nm formation of well N42 as an example, its Portable Water (GB5749-85), China National Standard for geothermal gradient is 3.76°C /100m, corresponding to a Natural Mineral Portable Water (GB8537-8) and China sand/mud ratio 25 to 75, but for Ng formation, its geothermal National Provisional Standard for Medical Treatment Mineral gradient decreases to 3.27°C /100m, with a sand : mud ratio Water, we can claim that groundwater from the upper section 43 :57. Another example is well X-2, the geothermal gradient of Nm, such as wells TG-3, HG-1 and DC-2 is portable in is up to 3.69 °C /100m corresponding to sand : mud ratio 40: terms of chemical and toxicological indexes. Geothermal 60, but its geothermal gradient drops to 2.62 °C /100m when water from Ordovician aquifer contains high silica and trace sand : mud ratio changes to 75:25. elements such as Li and can be utilized as mineral water for medical treatment. Since the thermal conductivity of the Pre-Cenozoic, Paleozoic and Pre-Paleozoic rocks are 1.4-1.6 times as high as the 6. ASSESSMENT OF THE RESOURCES Cenozoic formation, geothermal gradients in these rocks are about 2.5 to 3 °C /100m. There are some exceptions. In Taking the high potential sandstone reservoirs of the upper Jurassic, Lower Paleozoic and some coal-bearing strata, the Tertiary and limestone reservoirs of the lower Paleozoic into geothermal gradients can reach 3.5 to 4.0 °C /100m as account, we can calculate thermal water resources, both stored evidenced by logging data. But in some thick dolomite, and recoverable quantities (based on present exploiting carbonate, and granophyre formations, the gradients may be technology available) for each reservoir. The total area is ° as low as 1.5-2.0 C /100m. divided into 13 sectors with a total area of 3470 km2. 4.3 Local Geothermal Anomalies 6.1 Total Stored Thermal Water On a relative homogeneous geothermal background controlled Using the volumetric method, we have by structure patterns of uplifts and depressions, geothermal water migrates through faults in some places and creates local Q = A⋅b⋅ϕ (6-1) ° geothermal anomalies with geothermal gradients up to 7 C And /100m.